1. Field of the Invention
The present invention relates to a planar lighting device such as a back-lighting device in a liquid crystal display, for exiting illuminating light from a light exit surface of a light guide member by scattering rays of light incident on the light guide member from a light source.
2. Description of the Prior Art
As a planar lighting device such as a back-lighting device in a liquid crystal display, the planar lighting device has hitherto been well known which comprises a flat plate-like light guide member and a tubular light source disposed adjacent and in face-to-face relation with an end face of the light guide member, wherein rays of light from the light source are, after having entered the light guide member, scattered to allow illuminating light to emerge outwardly from the light exit surface of the light guide member to thereby uniformly illuminate the liquid crystal display from behind. In recent years, demand for increase of the size of the planar lighting device with an increased surface area of the light exiting surface of the light guide member has arisen intensively along with demand for reduction in thickness thereof and, therefore, the uniformity of brightness compatible with increase in size and reduction in thickness is required.
FIG. 13 illustrates the first planar lighting device according to the prior art, which comprises a light guide member 42 in the form of a transparent resin plate such as an acrylic resin plate having a high light transmissivity and two light sources 4 such as cold-cathode tubes each positioned adjacent a corresponding end face 46 of the light guide member 42 and covered by a reflective plate 27. This first prior art flat lighting device is such that while a reflecting surface 44 of the light guide member 42 is provided with a finely dotted irregular reflecting layer 29 formed by the use of a screen printing technique or a shot-blasting technique so that rays of light incident upon and propagating in the transparent resin plate 42 from the light sources 4 can be reflected to emerge outwardly from the light exit surface 45. The planar lighting device in which the irregular reflecting layer is formed on one surface of the transparent resin plate has an advantage in that it can be manufactured having a reduced thickness.
There is also known the second prior art planar lighting device which comprises, as shown in FIG. 14, a wedge-shaped light guide member 70 prepared from a transparent resin mixed with scattering particles having an index of refraction different from that of the transparent resin, and a tubular light source such as a cold-cathode tube disposed adjacent and in face-to-face relation with one end face 70a of the light guide member 70. The flat plate-like light guide member in which in place of the light guide member in the second-mentioned prior art planar lighting device wedge-shaped light guide sections are overlapped one above the other in the direction of thickness thereof as shown in FIG. 15. This third-mentioned prior art planar lighting device shown in FIG. 15 comprises a light guide plate 80 including a first light guide section 80A made of a transparent resin (a non-scattering light guide region) and a second light guide section 80B made of a transparent resin mixed with scattering particles having an index of refraction different therefrom (a scattering light guide region), and is disclosed in, for example, the U.S. Pat. No. 5,899,552.
Yet, the Japanese Laid-open Patent Publication No. 6-324330 discloses a fourth prior art planar lighting device comprising a light guide plate 90 including two wedge-shaped light guide sections 90A and 90B made of respective transparent resins containing a different concentration of scattering particles and, hence, having a different light scattering power, which wedge-shaped light guide sections 90A and 90B are overlapped one above the other in the thicknesswise direction thereof. In the third- and fourth-mentioned prior art planar lighting devices wherein the plural light guide plates having a different light scattering power are employed, as compared with the second-mentioned prior art planar lighting device, the light scattering power can be adjusted over the entire area of the light exit surface of the light guide plate and, therefore, the uniformity of brightness can easily be secured where the surface area of the light exit surface is relatively small.
The fifth prior art planar lighting device that satisfies the requirement of increase in size and reduction in thickness is illustrated in FIG. 17. This fifth prior art planar lighting device shown in FIG. 17 comprises a light guide member 50 formed to a shape similar to the prism-like hill corresponding to two light guide members 51 combined together, which light guide members 51 are similar to the wedge-shaped light guide sections 51A and 51B employed in the fourth prior art planar lighting device shown in FIG. 16, and two light sources 4 and 4 disposed adjacent and in face-to-face relation to opposite end faces 50a of the light guide member 50.
The planar lighting device formed in a hill-like shape with a polygonal pyramid and having a light source disposed adjacent and in face-to-face relation with each end face thereof is effective to provide a high luminance when rays of light are incident thereupon through the end faces. The hill-like shape may be a shape of, for example, a triangular pyramid and a square pyramid, and an extremely flattened hill-like shape can be obtained with respect to the requirement of the thickness being reduced. As a sixth prior art flat lighting device, a light guide member formed into, for example, a hill-like shape similar to the shape of a square pyramid is shown in FIG. 18A in a perspective representation. FIG. 18B is a cross-sectional representation of the light guide member 10 taken along the line IV--IV in FIG. 18A. This light guide member 10 includes a first light guide section (a non-scattering region) 11 having a recess of a shape similar to the inverted shape of a square pyramid and a second light guide section (a scattering region) 12 having a protrusion of a shape similar to the shape of a square pyramid.
As a method of making the light guide member used in the third to sixth prior art flat lighting devices, a method is known wherein, for example, two light guide sections are separately molded and are subsequently bonded together by the use of a bonding agent or the like.
Any of the first and second prior art planar lighting devices discussed above is suited where the light exit surface area is relatively small and, if the light exit surface is increased to increase the size of the planar lighting device, difficulty will arise in that a high brightness and a uniformity in luminance can no longer be secured. In other words, with the first prior art planar lighting device, in order to compensate for a phenomenon in which the brightness decreases with increase of the distance from the light source, the irregular reflectance is varied in unison with the light guiding distance (the distance from the end face of the light guide member), that is, the number of irregular reflecting layers is necessarily increased with increase of the distance away from the light source, and therefore, it is not easy to secure the uniformity in luminance. With the second prior art planar lighting device, since the single light source is employed, no high brightness can be obtained.
As regards the third prior art planar lighting device, as will be discussed in detail later, the first light guide section (a non-scattering light guide region) 80A and the second light guide section (a scattering light guide region) 80B are separately molded and are then bonded together by the use of a bonding agent or the like and, therefore, the workability is low since the both are bonded together after the molding, accompanied by increase in manufacturing cost. Also, since the bonding agent or the like is under at the interface, there is another problem in that it is difficult to secure flatness with high precision.
As regards the fourth prior art planar lighting device, since the light scattering power is varied by employing different concentrations of the scattering particles in the two light guide sections, the difference between those concentrations of the scattering particles in those light guide sections, that is, the gradient of the light scattering power, is limited to a small value. Where the planar lighting device is made large in size, it is necessary to increase the amount of light exiting therefrom as the distance increases away from the light source, but if the gradient of the light scattering power is small, the required amount of the light exiting therefrom can not be secured at a position distant from the light source, resulting in a problem associated with difficulty in securement of the uniformity of luminance. Also, where the concentration of the scattering particles in one of the light guide sections is reduced considerably, for example, down to 0.01 wt %, a problem will arise that such one of the light guide sections is difficult to manufacture.
Any one of the second to sixth prior art flat lighting devices involves such a problem that since due to the difference in angle between each of the reflecting surface and the light exit surface and the end face of the wedge-shaped light guide section, the rays of light from the light source incident upon the wedge-shaped light guide member repeatedly exit and reflect within the wedge shape and, hence, interfere with each other, resulting in occurrence of line-shaped bright spots at a portion of the light exit surface adjacent the light source. To eliminate the occurrence of the bright spot, it has hitherto been required to separately dispose a light reflecting member or a light absorbing member at a location adjacent the light source.
In addition, the second to sixth prior art flat lighting devices has an additional problem in that bright spots different from those discussed above tend to occur on a portion of the light exit surface adjacent the light source. By way of example, in the second prior art flat lighting device shown in FIG. 14, since the electrode portions 4a at the opposite ends of the light source (a cold cathode tube) 4 barely emit light, a sufficient amount of light will not enter the opposite side portions of the light guide member 70 facing the corresponding electrode portions 4a and, accordingly, the brightness of light emerging from respective portions of the light exit surface adjacent such side portions of the light guide member 70 tends to become insufficient, resulting in bright spots of the light emerging therefrom. Therefore, in order to avoid the occurrence of the bright spots, it is necessary for the light source 4 to be extended beyond the side surface 70b of the light guide member 70. Since extra portions of the light source so extended will occupy respective portions of the liquid crystal display where the display frame ought to have been and, therefore, the display frame cannot be narrowed.
On the other hand, as discussed hereinbefore, it has been known that the light guide member employed in any one of the third to sixth prior art flat lighting device is manufactured by, for example, molding the first and second light guide sections separately, followed by bonding of those light guide sections together by the use of a bonding agent. Since this method requires the bonding after the molding, the workability is not sufficient and the manufacturing cost is high. A further problem is involved in that since the bonding agent or the like is used at the interface, it is difficult to secure flatness with high precision. In particular, those problems are paramount when the light guide member is to be manufactured in a large size and a reduced thickness. In such case, in terms of productivity, the injection molding technique appears to be feasible rather than the conventional bonding technique. However, where the above described light guide member has a shape similar to the shape of a flattened hill so that it can be manufactured in a large size and a reduced thickness, formation of the light guide member by means of the conventional injection molding technique will bring about the following problems.
When it comes to the injection molding of the light guide member, it is a general practice to provide an inlet port 13 of a mold assembly at a location corresponding to an outer periphery of a display surface as shown in FIG. 18A. This is because if an inlet port 14 of a mold assembly is disposed at a location confronting the display surface, fine burrs (a trace of the inlet port) will be formed on the molded display surface, which burrs will form flaws that eventually adversely affects the display. For this reason, the necessity will occur to inject a resin from the outer periphery, but if a portion of the first light guide section 11 adjacent the bottom apex 11a of the recess in the first light guide section 11 is reduced in thickness in order for the light guide member to be eventually molded having a reduced thickness (which connotes reduction of the size of a gap in the mold assembly), the resin will hardly flow at that portion. Because of this, it is necessary for that portion of the bottom apex 11a to have a sufficient thickness to a certain extent, making it difficult to manufacture the light guide member having a reduced thickness.
Also, injecting the resin from the outer periphery requires increase of the distance over which the resin flows from one point of the outer periphery to the opposite point thereof. Therefore, the pressure and the load required to inject the resin are required to be high, resulting in a considerable increase of the strain in the resin which would in turn bring about unevenness in the resin. This in turn results in a problem associated with non-uniformity of the brightness.
Furthermore, when the second light guide section 12 is molded after the first light guide section 11 has been molded by the use of the injection molding technique, since the bottom apex 11a of the recess in the first light guide section 11 has a small thickness, heat will be evolved at a portion adjacent the bottom apex 11a by the effect of the injection molding pressure and, therefore, as the resin for the second light guide section 12 flows, the resin tends to become irregular at that portion adjacent the bottom apex 11a, resulting in a problem associated with non-uniformity of the brightness.